When you're looking at your list of apps in the App Manager portlet, there's an icon menu to the right of each app with the label "Actions". If you click that, and your app had already been approved, you can click "Deactivate". If you do this, you'll need to go through the review process once again to reactivate the app.

Removing an app from the Marketplace:1) Approved and published app Please above post.2) Submitted for review app Cancel Submission for you app, then discard all changes.3) Denied App Denied Apps do not appear on the Marketplace.4) Draft Discard your changes.